Over the past few years, the diesel engine has been relied upon increasingly to power automotive vehicles due to its fuel economy in comparison to conventional gasoline engines. Commercially available diesel engines for highway usage are conveniently classified into two categories, namely, those for use in light-duty vehicles and trucks, and those for use in heavy-duty vehicles. Light-duty vehicles and trucks are defined by the Environmental Protection Agency as passenger cars capable of seating twelve passengers or fewer, and light-duty trucks and all other vehicles under 8,501 pounds gross weight. This category includes most cars and pick-up trucks, mini-vans, and some special purpose vehicles. Heavy-duty vehicles are defined as all vehicles of 8,500 pounds or more gross weight. Heavy-duty vehicles are, typically, trucks, buses, vans and recreational vehicles.
Additionally, the diesel engine finds application in industrial settings such as storage facilities and underground mines, many of which have only limited ventilation. And, diesel engines find further significant utilization in diesel locomotives; industrial applications such as fork lift engines, auxiliary engines on large vehicles, generator and pump service, and in logging, mining, quarrying and oil field operations, as well as well-drilling equipment; construction applications, such as use in bulldozers, motor graders, tractors, scrapers, rollers and loaders; and agricultural applications, such as powering agricultural equipment.
However, despite its rising popularity, especially in the heavy-duty vehicle category, and although diesel engine exhaust (unlike that of gasoline engines) is relatively clean in respect of unburned hydrocarbon- and carbon-monoxide content, several significant difficulties are attendant upon use of the diesel engine. They stem essentially from the fact that diesel engine exhaust contains undesirably large amounts of solid particulate matter, for instance, in amounts at least thirty to fifty times greater than amounts present in the exhaust of a gasoline engine.
Typical solid particulate matter from diesel engine exhaust is made up of small, solid, irregularly shaped particles which are agglomerates of roughly spherical subunits. The particles often have high molecular weight hydrocarbons absorbed on their surfaces, and also may have a liquid coating; frequently, the particulate matter is a complex mixture of pure carbon and hundreds of organic compounds. The particulate is often extremely fine and light with a flour-like consistency. Size distribution ranges from very small single particles of about 0.01 microns to relatively large clusters in the range of 10-30 microns. Illustratively, the particles have a bulk density of 0.075 g/cm.sup.3 and have a surface area of 100 m.sup.2 /g. Generally speaking, the nature of solid particulate matter emitted from turbo-charged diesel engines is somewhat different from that of naturally aspirated diesel engines, the former tending to be smaller in size with much lower levels of retained organic compounds.
Unchecked, the aforementioned high level of solid particulate emission in diesel exhaust will continue to compound problems caused by the already high levels of total suspended particulates in the atmosphere, especially in urban areas. For example, as the diesel population increases it can be expected that there will be a decrease in visibility in major cities. Thus, the National Research Council estimates visibility loss in 1990 to be twenty percent in Los Angeles and fifty percent in Denver (Science, page 268, January 1982). Moreover, certain characteristic components of diesel exhaust particulate emissions have been identified as carcinogens; their presence in the atmosphere thus creates an evident and unacceptable health hazard. In this connection, the National Cancer Institute has published a study which showed that truck drivers operating diesel vehicles ran a risk of suffering bladder cancer up to twelve times that of the normal population (Wall Street Journal, Apr. 11, 1983).
Responding to the above-described situation, the Environmental Protection Agency has proposed a standard for particulate matter emission from diesel-powered light-duty vehicles of 0.6 g/mile, beginning with the 1987 model year; the agency has further proposed (for enforcement beginning with the 1990 model year) a standard for such emissions from diesel-powered heavy-duty vehicles of 0.25 g/bhp-hr (brake horsepower hour).
One of the options which is available to manufacturers of diesel engines and automotive vehicles for combating the aforementioned problem is deliberate suppression of power output in commercially produced diesel engines. Indeed, this technique is simply an extension of methods for controlling smoke and gaseous emissions as previously used by engine manufacturers. Specific examples of such technique are the methods used to minimize (1) acceleration smoke and (2) lugdown smoke.
Acceleration smoke is that generated during vehicle acceleration. It is caused by an undesirably higher fuel/air ratio and usually manifests itself as a short-duration, black puff. Lugdown smoke is generated during operation under a heavy load, for instance, during hill-climbing. It can conveniently be considered as full-load, steady-state smoke. Manufacturers compensate for these difficulties by mechanically limiting the amount of fuel injected under conditions at which the emissions are generated. Thus, smoke reduction is promoted at the cost of lost performance.
By the foregoing technique, engine manufacturers have made some headway in the endeavor to cut back the solid particulate emissions in the exhaust of such engines. But, although that technique has been somewhat helpful, it is not an adequate solution. That is, the aforementioned expedients are not effective to eliminate all solid particulate emission or even to decrease it to a desirably low level, unless power output is reduced to an unacceptably low level.
Several alternative possibilities for reducing emission levels have been investigated. Prominent among those possibilities are thermal and catalytic oxidation of particulate matter while it is still suspended in the exhaust stream, thermal oxidation of filter-trapped particulate matter, and catalytic oxidation of filter-trapped particulate matter. However, these possibilities generally have associated shortcomings which detract from their suitability as viable commercial solutions.
For example, thermal in-stream oxidation techniques require the provision to the exhaust stream of large amounts of heat energy which is typically unrecoverable. Catalytic in-stream oxidation requires devising a suitable means for introducing catalyst material into the exhaust stream, and preliminarily identification of an appropriate catalyst, both difficult problems which to date have resisted viable solution.
Other of the aforementioned possibilities involve use of a filter to remove solid particulate from a diesel engine exhaust stream. Use of filters has generated a relatively large amount of interest in the art. Experimentation has been conducted with a number of different types of filter materials, notably ceramic materials, stainless steel wire mesh, and the like. Filtration is, of course, a reasonably direct manner in which to remove particulate emission from an exhaust stream. However, use of filters is accompanied by significant difficulties resulting from the tendency of those filters to clog.
For many filtering materials particulate loading is an irreversible process insofar as once loading or clogging has reached a certain point, the filter element must be discarded and replaced since the initial restriction cannot be restored; for such filter elements, cleaning is ineffective. Even if clogging is not allowed to proceed to irreversibility, its occurrence leads to choking off of the exhaust flow through the filter. Since to be effective the filter must be positioned in the exhaust stream, filter-clogging tends to increase the pressure differential across the filter element and impede the exhaust operation--which detrimentally effects operation of the diesel engine. Accordingly, it is necessary, if filtration is to be a practical solution, to remove solid particulate matter which clogs exhaust flow filtering elements, i.e., regenerate the filter.
It is not surprising, therefore, that filter-regeneration, and the concomitant difficulty of disposing of the particulate matter removed from the filter, are central to the above-mentioned filtration techniques. But, while they address filter-regeneration and particulate disposal, the aforementioned techniques are not commercially attractive. For example, thermal and catalytic oxidation of filter-trapped particulate matter to regenerate the filter is problematical inasmuch as the space-, cost- and energy consumption-requirements which are entailed are substantial. These filtration techniques are no more acceptable than the direct, in-stream oxidation techniques which do not make use of filters.
As an indication of the direction the art has taken, see a recent survey and evaluation of the above-discussed proposals--Murphy et al., "Assessment of Diesel Particulate Control--Direct and Catalytic Oxidation", presented at the International Congress and Exposition, Cobo Hall, Detroit, Mich. (Feb. 23-27, 1981), SAE Technical Paper Series, No. 810,112--in which it is stated that the technique apparently holding greatest promise for removal of solid particulate matter from diesel engine exhaust is catalytic oxidation of filter-trapped particulate matter.
Another proposal for removal of solid particulate matter from diesel engine exhaust appears in U.K. Patent Application 2,097,283. That application discloses a method for filtration of exhaust flow, and corresponding apparatus, which involves use of ceramic filter material and no less than two filter zones which are alternately employed for filtering the exhaust stream of an internal combustion engine. The essence of that technique is the filtration of the exhaust stream with one filter zone while simultaneously regenerating the other filter zone by passing an appropriate fluid (e.g., air) through it, in a direction opposed to that of exhaust flow, in order to dislodge trapped solid particulate matter. That regeneration technique is known as backflushing. No quantification of backflushing time is given; it is apparent that backflushing is effected by continuous, relatively long-term passage of backflushing fluid through the filter zone being regenerated.
At a desired time the regenerated filter zone is inserted in the exhaust stream and the other filter zone is subjected to backflushing. In this manner, the filter zones are periodically rotated in an attempt to maintain effective engine operation during filtering.
In order to dispose of the solid particulate matter filtered from the exhaust, it is recycled to the engine for incineration. However, there is no teaching--for example, description of maximum residence time of particulate in the filter during exhaust flow or of the condition of the particulate during that residence time--in the above-identified U.K. Patent Application to guide the practitioner in preserving the solid particulate in a form such that it is suitable for efficient incineration in the engine.